This invention relates to helical resonators which are used as tuning elements to select signals within desired high frequency ranges. Such resonators include inductive elements in the form of a helical coil with the coil in proximity to a surrounding metallic enclosure. The grounded or low impedance end of the coil is directly connected to the metal enclosure. The high impedance opposite end is capacitively coupled to the enclosure. The resonant frequency of the resonator is a function of the physical dimensions of the coil, the capacitive structure and the distance between the high impedance end of the coil and the enclosure. In order to adjust the resonant frequency of the resonator, some prior art helical resonators have included a member such as a screw which is in electrical contact with the enclosure and which can be moved with respect to the high impedance end of the helical coil to thereby vary the equivalent capacitance of the composite structure.
In the prior art helical resonators, the structure has been found to be either inadequate and sometimes expensive for some applications. In one embodiment, a helical coil is wound around and supported by a ceramic form. The combination of the form and the coil is positioned in a metallic enclosure. One plate of a variable tuning capacitor thereof is comprised of an annular element, the position of which is adjustable with respect to the high impedance end of a coil by means of a threaded shaft in engagement with the enclosure. The other plate of this capacitor is comprised of a metal tab which is soldered to the high impedance end of the coil. This tab provides sufficient capacitive coupling between the coil and the annular element so that movement of the annular element can produce an appreciable change in the resonant frequency of the structure. However, when subjected to mechanical vibrations as may occur in a mobile receiver, the coil of this type of resonator has a tendency to vibrate like a coil spring about the ceramic form. This vibration produces corresponding changes in the inductance which undesirably modulate the radio frequency electrical signals passing therethrough. Further, the physical dimensions of the coil change in response to temperature variations tending to detune the resonator. Furthermore, the electrical connection between the tab and the high impedance end of the coil produce an electrical discontinuity in the coil thereby increasing insertion loss of the resonator. Although vibration problems and changes in dimension with temperature have been overcome by a ceramic coil form with grooves corresponding to the turns of the helical coil and the turns cemented to the form, the resulting resonator is relatively expensive and complex.
An attempt to reduce the cost of the resonators is described in U.S. Pat. No. 3,621,484 by D. L. Shult. In this arrangement, a helical resonator is comprised of a conductive housing which surrounds a coil mounted in a plastic form that has been injected molded thereabout. The coils imbedded in the plastic form prevent vibration of the coil. The windings at one end of the coil have a reduced diameter with respect to the rest of the diameter. The other end of the coil is grounded to a portion of the conductive housing. The wall adjacent the windings of reduced diameter form with the windings what is referred to as an equivalent capacitor. A tuning screw is passed through this wall into the hollow area enclosed by the windings of reduced diameter. Adjustment of the screw varies the resonant frequency of the helical resonator. Although this structure produces a low cost resonator, the Q (quality factor of reactive elements lowered by the resistance) of the resonator is diminished when tuning over a broad range of frequencies. When turning with the screw which passes through the windings as when tuning over a large tuning range, the screw causes a shorted turn effect which therefore loads the coil and causes a lowering of the Q. In mobile applications or in other such applications where the bandwidth is narrow, it is highly desirable that Q remain high. Therefore, a relatively inexpensive electrical resonator for applications where it is desired to maintain a high Q resonant circuit over a large tuning range, is in demand.